Cohomology of step functions under irrational rotations

This paper studies the solvability of the functional equationg(x+θ)=φ(x)g(x), given an irrationalθ and a step functionf mappingR/Z (with Lebesgue measure) to the unit circle. Results are applied to find parameterized families of representations of non-regular semi-direct product groups and to display irregularities in the uniform distribution of the sequenceZθ.     

Smooth cocycles for an irrational rotation

Explicit examples of smooth cocycles not cohomologous to constants are constructed. Necessary and sufficient conditions on the irrational numberθ are given for the existence of such cocycles. It is shown that, depending onθ, the set ofC ^r cocycles whose skew-product is ergodic is either residual or empty.     

The first return time properties of an irrational rotation

If an ergodic system has positive entropy, then the Shannon-McMillan-Breiman theorem provides a relationship between the entropy and the size of an atom of the iterated partition. The system also has Ornstein-Weiss' first return time property, which offers a method of computing the entropy via an orbit. We consider irrational rotations which are the simplest model of zero entropy. We prove that almost every irrational rotation has the analogous properties if properly normalized. However there are some irrational rotations that exhibit different behavior.

     

On discrepancies of irrational rotations: an approach via rational rotation

Setokuchi and Takashima (Unif Distrib Theory (2) 9:31–57, 2014) and Setokuchi (Acta Math Hung, [11]) gave refinements of estimates for discrepancies by using Schoissengeier’s exact formula. Mori and Takashima (Period Math Hung, [7]) discussed the distribution of the leading digits of \(a^n\) by approximating irrational rotations by “rational rotations”. We apply here their methods to the estimation of discrepancies. We give much more accurate estimates for discrepancies by simple direct calculations, without using Schoissengeier’s formula. We show that the initial segment of the graph of discrepancies of irrational rotations with a single isolated large partial quotient is linearly decreasing, provided we observe the discrepancies on a linear scale with suitable step. We also prove that large hills, caused by single isolated large partial quotients, will appear infinitely often.     

Skew products over the irrational rotation

Here we give conditions on a class of functions defining skew product extensions of irrational rotations on T which ensure ergodicity. These results produce extensions of the work done by P. Hellekalek and G. Larcher [HL1] and [HL2] to the larger class of functions which are piecewise absolutely continuous, have zero integral and have a derivative which is Riemann integrable with a non-zero integral. Supported by SERC Grant No. 85318881.     

Convergence of random walks on the circle generated by an irrational rotation

Fix . Consider the random walk on the circle which proceeds by repeatedly rotating points forward or backward, with probability , by an angle . This paper analyzes the rate of convergence of this walk to the uniform distribution under ``discrepancy' distance. The rate depends on the continued fraction properties of the number . We obtain bounds for rates when is any irrational, and a sharp rate when is a quadratic irrational. In that case the discrepancy falls as (up to constant factors), where is the number of steps in the walk. This is the first example of a sharp rate for a discrete walk on a continuous state space. It is obtained by establishing an interesting recurrence relation for the distribution of multiples of which allows for tighter bounds on terms which appear in the Erdös-Turán inequality.

     

Wandering vectors for irrational rotation unitary systems

An abstract characterization for those irrational rotation unitary systems with complete wandering subspaces is given. We prove that an irrational rotation unitary system has a complete wandering vector if and only if the von Neumann algebra generated by the unitary system is finite and shares a cyclic vector with its commutant. We solve a factorization problem of Dai and Larson negatively for wandering vector multipliers, and strengthen this by showing that for an irrational rotation unitary system , every unitary operator in is a wandering vector multiplier. Moreover, we show that there is a class of wandering vector multipliers, induced in a natural way by pairs of characters of the integer group , which fail to factor even as the product of a unitary in and a unitary in . Incomplete maximal wandering subspaces are also considered, and some questions are raised.

     

Gauss polynomials and the rotation algebra

Newton's binomial theorem is extended to an interesting noncommutative setting as follows: If, in a ring,ba=ab with commuting witha andb, then the (generalized) binomial coefficient arising in the expansion

Solution of an algebraic equation using an irrational iteration function

It is proved that, for the choice z _n ^[n] = ?a ₁ of the initial approximation, the sequence of approximations z _n ^[i+1] = φ _n (z _n ^[i] ), [i] = 0, 1, 2, ..., of a solution of every canonical algebraic equation with real positive roots which is of the form $$P_n (z) = z^n + a_1 z^{n - 1} + a_2 z^{n - 2} + \ldots + a_n = 0, n = 1,2, \ldots ,$$ where the sequence is generated by the irrational iteration function φ _n (z) = (z ⁿ ? P _n (z))^1/n , converges to the largest root z _n . Examples of numerical realization of the method for the problem of determining the energy levels of electron systems of a molecule or a crystal are presented. The possibility of constructing similar irrational iteration functions in order to solve an algebraic equation of general form is considered.     

The Coherent Cohomology Ring of an Algebraic Group

Let G be a group scheme of finite type over a field, and consider the cohomology ring H ^*(G) with coefficients in the structure sheaf. We show that H ^*(G) is a free module of finite rank over its component of degree 0, and is the exterior algebra of its component of degree 1. When G is connected, we determine the Hopf algebra structure of H ^*(G).     

Hosoya polynomials under gated amalgamations

An induced subgraph H of a graph G is gated if for every vertex x outside H there exists a vertex x^′ inside H such that each vertex y of H is connected with x by a shortest path passing through x^′. The gated amalgam of graphs G₁ and G₂ is obtained from G₁ and G₂ by identifying their isomorphic gated subgraphs H₁ and H₂. Two theorems on Hosoya polynomials of gated amalgams are provided. As their applications, explicit expressions for Hosoya polynomials of hexagonal chains are obtained.